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Metabolic middle ground

Editor's Introduction

Evidence for mesothermy in dinosaurs

For several decades, researchers have struggled to determine whether dinosaurs had energetic systems closer to those of rapidly metabolizing endotherms like mammals and birds, or to those of slower, ectothermic reptiles that do not internally regulate their body temperature. The problem was that it was difficult to estimate the metabolic rates of species that no longer exist.

Grady and colleagues developed a new approach to predict the metabolic rates of 21 dinosaur species, showing that dinosaur metabolism was neither fast nor slow, but somewhere in the middle. Similar to a handful of living species, dinosaurs were mesotherms that used their metabolism to internally regulate their body temperature without keeping it at a specific level.

Image. Leather back turtles, great white sharks, and bluefin tuna are living examples of mesotherms.(Image courtesy of U.S. Fish and Wildlife Service Headquarters and Elias Levy through Wikimedia Commons.)

2. Endotherms have faster growth rates than mesotherms or ectotherms of the same size.

3. Tuna, mesotherms.

4. a. 1 and 100 b. 10,000 and 100,000

5. 104 x or 10,000 times

Fig. 2. Vertebrate growth energetics. (A) Relationship between growth and resting metabolic rate for vertebrates. The dashed line is the theoretical prediction; the solid line represents an OLS fitted regression with 95% confidence bands. (B) Predicted energetics of dinosaurs. Dinosaur rates (open squares) from Eq. 2 are plotted on the theoretical line. The ranges in metabolic rates occupied by extant endotherms, mesotherms, and ectotherms are indicated by color.

Panel A questions

A very large ectotherm is likely to have a higher metabolic rate than a very small endotherm simply because there are more cells in the animal. Figure 2 compares metabolic rate and growth rates per gram of mass. These are mass-independent rates.

A1. Compare and contrast mass-independent metabolic rates and growth rates of endotherms to that of ectotherms.

A2. Which two groups of endotherms do not follow the theoretical prediction (solid line)?

A3. Describe why a high metabolic rate would support an endotherm's thermoregulation. (Hint: Apply the second law of thermodynamics.)

Panel B questions

Let's look more closely at dinosaurs (panel B).

B1. Which variable can be measured in dinosaurs, and how is it measured?

B2. If an organism has a mass-independent growth rate of 10-2 (g1/4d-1) what do you predict its mass-independent metabolic rate would be?

B3. If an organism had a mass-independent growth rate of 10-3, what thermoregulation type would you predict it to be?

Panel A answers

A1. Regardless of the size of the organism, endotherms have higher mass-independent metabolic rates and growth rates than ectotherms.

A2. Altricial birds and primates, toothed whales. ("Altricial" means born in an undeveloped state, requiring parental care.) Note that birds typically have very high metabolic rates relative to their mass, whereas primates and large mammals have slow growth rates for their size. This slow growth rate is often associated with large brain size and low juvenile mortality that would not be associated with dinosaurs.

A3. According to the second law of thermodynamics, heat is a byproduct of energy conversion. A faster metabolic rate would result in more heat generated through metabolism. This heat would increase the organism's internal body temperature to a level higher than the ambient (surrounding environmental) temperature.

Panel B answers

B1. Growth rate can be measured using the width of growth rings seen in fossilized dinosaur bones. For more information, see: http://www.pbs.org/wgbh/nova/nature/t-rex.html

B2. Mass-independent metabolic rate would be approximately 10-2 W g-3/4.

A3. How would the amount of food required to support a 10-gram endotherm compare to the amount needed to support a 10-gram ectotherm?

A4. What would be the advantage and disadvantage of being an endotherm versus an ectotherm?

Answers A

A1. As the mass of an animal increases, so does its metabolic rate. A large organism has more cells, resulting in a higher energy demand.

A2. At any given size (mass) endotherms have a higher metabolic rate than ectotherms. Mesotherms have an intermediate metabolic rate.

A3. To support a higher metabolic rate, the 10-gram endotherm will need to eat more food than the 10-gram ectotherm.

A4. Endotherm: Advantage: can use heat generated through a high metabolic rate to maintain an optimal body temperature despite fluctuations in the surrounding temperature. This allows endotherms to live in cooler climates. Disadvantage: A high metabolic rate will have a higher caloric cost, requiring the animal to eat more food.

Ectotherm: Advantage: A lower metabolic rate means a lower caloric demand, which, in turn, allows the animal to spend less time on hunting and foraging for food. Disadvantage: The animal has to use behavioral strategies, such as sun basking, to maintain optimal body temperature. Moreover, the metabolic rate decreases when the ambient temperature falls. As a result, the animal is much less active at cooler temperatures. Ectotherms may therefore be restricted to warmer climates, which limits their overall distribution.

Questions (predicted metabolic rate)

Panel B shows the metabolic rate of dinosaurs as gray squares and the dashed line. Recall that the metabolic rate of an extinct species cannot be directly measured.

B1. How did the researchers determine the predicted metabolic rate of the dinosaurs?

B2. How does the graph in panel A contribute to the validity of the dinosaur prediction?

B3. How do you think a mesothermic metabolism benefited the dinosaurs?

Answers B

B1. To determine the metabolic rate of different dinosaur species, the researchers used two of the available pieces of information in the fossil record: adult mass and maximum growth rate (bone growth rings).

B2. Panel A illustrates that using maximum growth rate is a reliable way to accurately predict the metabolic rate in extant (living) species. It should therefore also be a reliable approach to predict the metabolic rate of extinct species.

B3. Because of their large size, dinosaurs would have required an enormous amount of food for growth and being active. A mesothermic metabolic rate could have allowed them to use some metabolic heat to maintain a higher body temperature, allowing them to increase their range and maintain activity level when the ambient temperature fluctuated. Their large size and insulating feathers may have been enough to maintain an optimal body temperature without the high caloric cost of a true endothermic fast metabolic rate.

Phylogeny

This figure shows a phylogenetic tree of the animal groups compared by the researchers. Phylogenetic trees outline the evolutionary relationships of organisms. These relationships are determined by the number of homologies shared between the organisms that are compared.

Homologies are inherited features such as similar anatomical structures, DNA and protein sequences, and metabolic processes that provide evidence for common ancestry. The more homologies there are between two species, the more recently they shared a common ancestor. The most closely related species will share the longest branch before separating from each other.

This link provides more information regarding how DNA sequences are used to construct phylogenetic trees. http://www.hhmi.org/biointeractive/creating-phylogenetic-trees-dna-sequences

2. Which likely evolved first in the ancestors of modern birds: feathers or a high metabolic rate to support endothermy? Provide evidence to support your claim.

3. Is endothermy a shared homology between birds (paleognathae) and mammals (placentalia)? Why or why not?

Phylogeny answers

1. Dinosaurs share the longest branch with birds (paleognathae). This means they share the most recent common ancestor with birds, followed by reptiles, and mammals. They are most distantly related to fish.

2. Feathers evolved prior to the high metabolic rate of birds. Tyrannosaurs, Trodon, and Archaeopteryx were feathered but had slower growth rates, indicating a slower metabolic rate characteristic of a mesothermic organism.

3. Endothermy likely evolved independently in birds and mammals. Birds share more homologies and therefore more recent ancestors with reptiles, which have the metabolic rates and growth rates of ectotherms and mesotherms, than with mammals.

Endothermy in birds and mammals is an example of convergent evolution in which similar selective pressures have results in the selection for and evolution of similar characteristics.

At the largest body masses, the growth rates of the largest dinosaurs and mammals overlap (Fig. 1B). This pattern is driven by two factors. First, dinosaurs have a relatively high slope (αOLS = 0.82, but αPIC = 0.76). This value is consistent with suggestions of thermal inertia for larger taxa; the removal of sauropods yields a reduced OLS slope of 0.77. Second, significantly reduced growth rates are observed in several large mammalian taxa, particularly primates, elephants, and toothed whales, whereas small shrews and rodents have relatively high rates, leading to a low overall slope for placental mammals (αOLS = 0.64, αPIC = 0.63; table S2 and fig. S11). The slow growth of many large endothermic mammals is associated with large brain size and low juvenile mortality (19, 20); this is unlikely to be relevant to most dinosaurs.